Crocs or sandals are great for the water rides. I usually take an pair of sandals or aqua-shoes for the water rides in a backpack.
I usually opt for flip flops when I head to Universal, as I usually just see the shows and catch just a few rides, so I don't do too much walking and my flip flops are quite comfortable.
In one where there are no water rides (Disney or not), I wear either water shoes or sneaks - it depends on mood.
I would vote crocs too (like all y'all) but dang... it just doesn't match well with my mini skirts.
I try to where something that I would wear out in public, usually nice shoes, khaki/cargo shorts, collared t-shirt, and a baseball hat. I'm uncomfortable wearing wife-beaters, sandals, swimsuits, or other casual attire in crowds that large. I like to appear somewhat presentable.
I have been intrigued by Crocs, but since I wear size 14 shoes, I have never seen them in my size. So when I go to a theme park, I wear a nice pair of broken in sneakers. I may bring a change of socks with me too if I can remember. You know, for those times you ride Popeye at Islands of Adventure but don't have enough time to get your shoes off. It happens!
a few years ago I was at the Islands of Adventure with my brother-in-law who wore some new Adidas sandals (the vinly / rubbery kind). By the end of the day he was walking around barefoot because he found it more comfortable, that is how bad the sandals got. I'm telling you, sneakers are the only way to go.